Method and apparatus for sensing environmental parameters using wireless sensor(s)

ABSTRACT

A passive radio frequency identification (RFID) sensor is provided. This passive RFID sensor includes an inductive loop, a processing module, and a wireless communication module. The inductive loop has an inductive loop impedance that may vary with an environment in which the inductive loop is placed. The processing module couples to the inductive loop and has a tuning module that may vary a reactive component impedance coupled to the inductive loop in order to change a system impedance. The system impedance includes both the inductive loop impedance and the reactive component impedance. The tuning module then produces an impedance value representative of the reactive component impedance. A memory module may store the impedance value which may then later be communicated to an RFID reader via the wireless communication module. The RFID reader may then exchange the impedance value representative of the reactive components of impedance with the RFID reader such that the RFID reader or another external processing unit may process the impedance value in order to determine environmental conditions at the inductive loop. These environmental conditions may include but are not limited to temperature, humidity, wetness, or proximity of the RFID reader to the passive RFID sensor.

CROSS-REFERENCE TO RELATED PATENTS

This Patent Application is claiming priority under 35 USC §120 as acontinuation-in-part patent application of co-pending patent applicationentitled “Method and Apparatus for Sensing Environment Using a WirelessPassive Sensor”, having a filing date of 18 Apr. 2014 and a Ser. No.14/256,877.

This application further claims priority under 35 USC §119(e) to thefollowing U.S. Provisional Patent Applications:

-   -   1. U.S. Provisional Application Ser. No. 62/004,941, filed 30        May 2014, (Entitled: “Pressure/Proximity Sensors reference        design”);    -   2. U.S. Provisional Application Ser. No. 62/004,943, filed 30        May 2014, (Entitled: “Method and Apparatus for Varying an        Impedance”);    -   3. U.S. Provisional Application Ser. No. 62/011,116, filed 12        Jun. 2014, (Entitled: “Method and Apparatus for Sensing Water        Level Using Wireless Sensor(s)”);    -   4. U.S. Provisional Application Ser. No. 62/131,414, filed 11        Mar. 2015, (Entitled: “Method and Apparatus for Variable        Capacitor Control”);        The above-referenced pending U.S. patent application Ser. No.        14/256,877 claims priority:    -   1. under 35 USC §120 as a continuation-in-part patent        application to then co-pending patent application entitled        “Method and Apparatus for Detecting RF Field Strength”, having a        filing date of 14 Aug. 2011, and a Ser. No. 13/209,420, now        issued U.S. Pat. No. 8,749,319;    -   2. under 35 USC §120 as a continuation-in-part patent        application to then co-pending patent application entitled        “Method and Apparatus for Detecting RF Field Strength”, having a        filing date of 14 Aug. 2011, and a Ser. No. 13/209,425, now        issued U.S. Pat. No. 9,048,819;    -   3. under 35 USC §120 as a continuation-in-part patent        application to co-pending patent application entitled        “Roll-To-Roll Production of RFID Tags”, having a filing date of        9 May 2012, and a Ser. No. 13/467,925;    -   4. under 35 USC §119(e) to U.S. Provisional Application Ser. No.        61/814,241, filed 20 Apr. 2013;    -   5. under 35 USC §119(e) to U.S. Provisional Application Ser. No.        61/833,150, filed 10 Jun. 2013;    -   6. under 35 USC §119(e) to U.S. Provisional Application Ser. No.        61/833,167, filed 10 Jun. 2013;    -   7. under 35 USC §119(e) to U.S. Provisional Application Ser. No.        61/833,265, filed 10 Jun. 2013;    -   8. under 35 USC §119(e) to U.S. Provisional Application Ser. No.        61/871,167, filed 28 Aug. 2013;    -   9. under 35 USC §119(e) to U.S. Provisional Application Ser. No.        61/875,599, filed 9 Sep. 2013;    -   10. under 35 USC §119(e) to U.S. Provisional Application Ser.        No. 61/896,102, filed 27 Oct. 2013;    -   11. under 35 USC §119(e) to U.S. Provisional Application Ser.        No. 61/929,017, filed 18 Jan. 2014; and    -   12. under 35 USC §119(e) to U.S. Provisional Application Ser.        No. 61/934,935, filed 3 Feb. 2014.        The above-referenced then pending U.S. patent application Ser.        No. 13/209,420 claims priority:    -   1. under 35 USC §120 as a continuation-in-part patent        application to then co-pending patent application entitled        Method And Apparatus For Varying An Impedance, having a filing        date of 1 Aug. 2009, and a Ser. No. 12/462,331, now issued U.S.        Pat. No. 8,081,043;    -   2. under 35 USC §119(e) to U.S. Provisional Application Ser. No.        61/428,170, filed 29 Dec. 2010; and    -   3. under 35 USC §119(e) to U.S. Provisional Application Ser. No.        61/485,732, filed 13 May 2011.        The above-referenced then pending U.S. patent application Ser.        No. 13/209,425 claims priority:    -   1. under 35 USC §120 as a continuation-in-part patent        application to then co-pending patent application entitled        Method And Apparatus For Varying An Impedance, having a filing        date of 1 Aug. 2009, and a Ser. No. 12/462,331, now issued U.S.        Pat. No. 8,081,043;    -   2. under 35 USC §119(e) to U.S. Provisional Application Ser. No.        61/428,170, filed 29 Dec. 2010; and    -   3. under 35 USC §119(e) to U.S. Provisional Application Ser. No.        61/485,732, filed 13 May 2011.        The above-referenced pending U.S. patent application Ser. No.        13/467,925 claims priority:    -   1. under 35 USC §120 as a continuation-in-part patent        application to then co-pending patent application entitled        Method and Apparatus for Detecting RF Field Strength, having a        filing date of 14 Aug. 2011, and a Ser. No. 13/209,425, now        issued U.S. Pat. No. 9,048,819;    -   2. under 35 USC §119(e) to U.S. Provisional Application Ser. No.        61/485,732, filed 13 May 2011;        The above-referenced then pending U.S. patent application Ser.        No. 12/462,331 claims priority:    -   1. under 35 USC §121 as a divisional patent application to then        co-pending patent application entitled Method And Apparatus For        Varying An Impedance, having a filing date of 18 Nov. 2006, and        a Ser. No. 11/601,085, now issued U.S. Pat. No. 7,586,385;        The above identified U.S. Provisional Patent Applications, U.S.        patent applications, and priority application lineage and their        subject matter are expressly incorporated by reference in their        entirety and made part of the present U.S. Utility Patent        Application for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to sensing a detectableenvironmental condition, and, in particular, to sensing a detectableenvironmental condition in a passive RFID system.

2. Description of the Related Art

In general, in an RF communication system, a single antenna structure isadapted to receive signals, the carrier frequencies (“f_(C)”) of thesesignals can vary significantly from the resonant frequency (“f_(R)”) ofthe antenna. The mismatch between f_(C) and f_(R) results in loss oftransmitted power. In some applications, this may not be of particularconcern, but, in others, such as in RF identification (“RFID”)applications, such losses are of critical concern. For example, in apassive RFID tag, a significant portion of received power is used todevelop all of the operating power required by the RFID tag's electricalcircuits. In such an application, a variable impedance circuit can beemployed to shift the f_(R) of the tag's receiver so as to better matchthe f_(C) of the transmitter of the system's RFID reader. A singledesign that is useful in all systems is precluded by the lack ofstandards as to appropriate RFID system frequencies, and, the breadth ofthe available frequency spectrum is quite broad: Low-Frequency (“LF”),including 125-134.2 kHz and 140-148.f kHz; High-Frequency (“HF”) at13.56 MHz; and Ultra-High Frequency (“UHF”) at 868-928 MHz. Compoundingthis problem is the fact that system manufacturers cannot agree on whichspecific f_(C) is the best for specific uses, and, indeed, to preventcross-talk, it is desirable to allow each system to distinguish itselffrom nearby systems by selecting different f_(C) within a defined range.

Attempts have been made to improve the ability of the tag's antenna tocompensate for system variables, such as the materials used tomanufacture the tag. However, such structural improvements, whilevaluable, do not solve the basic need for a variable impedance circuithaving a relatively broad tuning range.

Shown in FIG. 1 is an ideal variable impedance circuit 100. Circuit 100comprised of a variable inductor 102, a variable capacitor 104 and avariable resistor. When used as a tank in a resonant system, the circuit100 exhibits a quality factor (“Q”) of:

$\begin{matrix}{Q = {\frac{f_{R}}{\Delta_{f}} = {\frac{1}{R}\sqrt{\frac{L}{C}}}}} & \lbrack 1\rbrack\end{matrix}$

where: Q=the quality factor of circuit 100;

f_(R)=the resonant frequency of circuit 100, measured in hertz;

Δf=the bandwidth of circuit 100, measured in hertz at −3 db

R=the resistance of resistor, measured in ohms;

L=the inductance of variable inductor 102, measured in henries; and

C=the capacitance of capacitor, measured in farads.

In such a system, the resonant frequency, f_(R), of circuit 100 is:

$\begin{matrix}{f_{R} = \frac{1}{2\pi \sqrt{LC}}} & \lbrack 2\rbrack\end{matrix}$

As is well known, the total impedance of circuit 100 is:

$\begin{matrix}{Z = \frac{z_{L}z_{C}}{z_{L} + z_{C}}} & \lbrack 3\rbrack\end{matrix}$

where: Z=the total impedance of circuit 100, measured in ohms;

Z_(L)=the impedance of variable inductor 102, measured in ohms; and

Z_(C)=the impedance of capacitor, measured in ohms.

As is known, the relationship between impedance, resistance andreactance is:

Z=R+jX   [4]

where: Z=impedance, measured in ohms;

R=resistance, measured in ohms;

j=the imaginary unit √{square root over (−1)}; and

X=reactance, measured in ohms.

In general, it is sufficient to consider just the magnitude of theimpedance:

|Z|=√{square root over (R ² +X ²)}  [5]

For a purely inductive or capacitive element, the magnitude of theimpedance simplifies to just the respective reactances. Thus, forvariable inductor 102, the reactance can be expressed as:

X _(L)=2πfL   [6]

Similarly, for capacitor, the reactance can be expressed as:

$\begin{matrix}{X_{C} = \frac{1}{2\pi \; {fC}}} & \lbrack 7\rbrack\end{matrix}$

Because the reactance of variable inductor 102 is in phase while thereactance of capacitor is in quadrature, the reactance of variableinductor 102 is positive while the reactance of capacitor is negative.Accordingly, a desired total impedance can be maintained if a change ininductive reactance is offset by an appropriate change in capacitivereactance.

Within known limits, changes can be made in the relative values ofvariable inductor 102, capacitor, and resistor to adjust the resonantfrequency, f_(R), of circuit 100 to better match the carrier frequency,f_(C), of a received signal, while, at the same, maximizing Q.

In many applications, such as RFID tags, it may be economicallydesirable to substitute for variable inductor 102 a fixed inductor 202,as in the variable tank circuit 200 shown in FIG. 2. In general, inorder to maximize Q in circuit 200.

The amplitude modulated (“AM”) signal broadcast by the reader in an RFIDsystem will be electromagnetically coupled to a conventional antenna,and a portion of the current induced in a tank circuit is extracted by aregulator to provide operating power for all other circuits. Oncesufficient stable power is available, the regulator will produce, e.g.,a power-on-reset signal to initiate system operation. Thereafter, themethod disclosed in the Related References, and the associatedapparatus, dynamically varies the capacitance of a variable capacitorcomponent of the tank circuit so as to dynamically shift the f_(R) ofthe tank circuit to better match the f_(C) of the received RF signal,thus obtaining maximum power transfer in the system.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention are directed to systems and methodsthat are further described in the following description and claims.Advantages and features of embodiments of the present invention maybecome apparent from the description, accompanying the drawings andclaims.

Accordingly, the above problems and difficulties are obviated byembodiments of the present invention which provide an RF-basedenvironmental sensing system comprising one or more special antennaarrangements, and an RF transceiver. In this embodiment, the antennaarrangement comprises: an antenna having an antenna impedance; and atransmission line operatively coupled to the antenna and adaptedselectively to modify the antenna impendence. Further, the RFtransceiver comprises: a tank circuit operatively coupled to the antennaand having a selectively variable impedance; and a tuning circuitadapted to dynamically vary the impedance of the tank circuit, and todevelop a first quantized value representative of the impedance of thetank circuit, wherein the first quantized value is a function of themodified antenna impedance.

Further, we provide a method for operating the first embodimentcomprising the steps of: exposing the transmission line to a selectedenvironmental condition; dynamically varying the impedance of the tankcircuit substantially to match the modified antenna impedance; and usingthe first value to sense the environmental condition.

Another embodiment of the present disclosure provides an environmentalsensing method for use in an RF system comprising the steps of:calibrating an RF sensor by developing a first calibration valueindicative of an absence of a detectable quantity of a substance and asecond calibration value indicative of a presence of the detectablequantity of the substance; installing the sensor in a structure;exposing the structure to the substance; interrogating the sensor toretrieve a sensed value; and detecting the presence of the substance inthe structure as a function of the sensed value relative to the firstand second calibration values.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

My invention may be more fully understood by a description of certainpreferred embodiments in conjunction with the attached drawings inwhich:

FIG. 1 is an ideal variable impedance circuit;

FIG. 2 is a second variable impedance circuit;

FIG. 3 illustrates, in block diagram form, an RF receiver circuit havinga field strength detector constructed in accordance with an embodimentof the present disclosure;

FIG. 4 illustrates, in block diagram form, a field strength detectorcircuit constructed in accordance with an embodiment of the presentdisclosure;

FIG. 5 illustrates, in block schematic form, a more detailed embodimentof the field strength detector circuit shown in FIG. 4;

FIG. 6 illustrates, in flow diagram form, the sequencing of operationsin the field strength detector circuit shown in FIG. 5;

FIG. 7 illustrates, in graph form, the response of the field strengthdetector circuit shown in FIG. 5 to various conditions;

FIG. 8 illustrates, in block schematic form, an RF receiver circuitconstructed in accordance with another embodiment of the presentdisclosure;

FIG. 9 illustrates, in flow diagram form, the sequencing of theoperations in the RF receiver circuit shown in FIG. 8;

FIG. 10 illustrates, in block schematic form, an alternativerepresentation of the impedance represented by the antenna and the tankcircuit of the exemplary RFID receiver circuit;

FIG. 11 illustrates, in block schematic form, an alternative exemplaryembodiment of the field strength detector circuit shown in FIG. 5;

FIG. 12 illustrates, in block schematic form, an alternative exemplaryembodiment of the field strength detector circuit shown in FIG. 5;

FIG. 13 illustrates, in block schematic form, an exemplary RFIDsub-system containing tag and reader;

FIG. 14 illustrates, in flow diagram form, the sequencing of theoperations in developing a reference table associating tank tuningparameters with system frequency;

FIGS. 15A and 15B illustrate an RF system constructed in accordance withone embodiment of the present disclosure to sense environmentalconditions in a selected region surrounding the system;

FIG. 16 illustrates, in perspective, exploded view, one possibleconfiguration of an antenna and tail arrangement adapted for use in thesystem of FIGS. 15A and 15B;

FIG. 17A illustrates in top plan view a fully assembled antenna;

FIG. 17B and FIG. 17C illustrate, in cross-section, the several layerscomprising a head and a tail portion, respectively, of the antenna;

FIG. 17D through FIG. 17G illustrate, in plan view, the several separatelayers of the antenna as shown in FIG. 17B and FIG. 17C;

FIG. 17H illustrates, in partial in partial plan view, a close-updepiction of a central, slot portion of the antenna of FIG. 17A (asnoted in FIG. 17E) showing in greater detail the construction of antennaelements to which an RFID tag may be attached;

FIG. 18 illustrates, in flow diagram form, the sequencing of theoperations in detecting the presence of a contaminant using, e.g., theantenna of FIG. 17A;

FIG. 19A illustrates, in plan view, the top layer of the antenna afterplacement of the RFID tag die but before folding along fold lines 1 and2; and

FIG. 19B illustrates, also in plan view, the bottom layer of the antennaas shown in FIG. 19A;

FIG. 20 is depiction of an antenna inlay that may be used m accordancewith embodiments of the present disclosure;

FIG. 21 is a block diagram of a RFID wireless solution provided byembodiments of the present disclosure;

FIG. 22 is a block diagram of one arrangement of smart sensors and adata processing unit 2202 in accordance with embodiments of the presentdisclosure;

FIGS. 23A and 23B are views of an RFID seal tag with an antenna sealedby an induction seal in accordance with an embodiment of the presentdisclosure;

FIGS. 24A and 24B depict different antenna inlays that may be used inaccordance with embodiments of the present disclosure;

FIG. 25 provides an illustration of an antenna arrangement in accordancewith embodiments of the present disclosure;

FIG. 26 provides an illustration of an antenna arrangement in accordancewith embodiments of the present disclosure;

FIGS. 27A and 27B are graphs of the conjugate match factor (CMF) wherethe passive RFID sensor tags have a thickness of 0.03 and 0.05 inchesthick tags with an MMS value of 0 in accordance with another embodimentof the present disclosure;

FIGS. 28A and 28B provide graphs of the antenna impedance for thepreviously depicted antenna of FIG. 25 in accordance with anotherembodiment of the present disclosure;

FIGS. 29A and 29B provide sensitivity graphs for varying thicknessesassociated with the antenna's impendence, directivity, and radiationefficiency values are used to predict the RF sensitivity and read rangeof the RFID tag in accordance with embodiments of the presentdisclosure;

FIGS. 30A and 30B are illustrations of an antenna arrangement inaccordance with another embodiment of the present disclosure.

FIGS. 31 and 32 are views of an RFID pressure sensing tag in accordancewith an embodiment of the present disclosure;

FIG. 33 is a view of an RFID moisture or humidity sensing tag inaccordance with an embodiment of the present disclosure;

FIGS. 34A and 34B are views of a folded RFID tag comprising a radiatingelement in accordance with an embodiment of the present disclosure;

FIG. 35 is a block diagram of one arrangement of a self-tuning engine tosupport the reporting of several stimuli with multiple passive RFIDsensors using an antenna impedance sensing mechanism in accordance withembodiments of the present disclosure;

FIG. 36 is a block diagram of a self-tuning engine in accordance withembodiments of the present disclosure;

FIGS. 37A and 37B are representations of MOS Varactors utilized byembodiments of the present disclosure;

FIGS. 38A and 38B are representations of the CV curve behavior of theMOS Varactors in FIGS. 37A and 37B respectively;

FIG. 39 illustrates an embodiment of the self-tuning engine provided byembodiments of the present disclosure along with the varactors that aredriven by the tuning circuitry (referred to as MMS engine);

FIG. 40 is a graph of a Varactor's CV curves for different VSD voltagesprovided by embodiments of the present disclosure;

FIG. 41 illustrates another embodiment of the self-tuning engineprovided by embodiments of the present disclosure along with thevaractors that are driven by the tuning circuitry;

FIG. 42 is a graph of a CV curve for the varactor when S/D voltages areswept in accordance with embodiments of the present disclosure;

FIG. 43 provides a flow chart of one embodiment of the presentdisclosure;

FIG. 44 is a block diagram of an embodiments of the present disclosurethat provides a passive tire pressure monitoring system;

FIG. 45 is a block diagram of a moisture sensor used to monitor thelevel within a reservoir or tank in accordance with embodiments of thepresent disclosure;

FIG. 46 provides experiment results that illustrate the relation betweenfluid height and a sensor code generated by a sensor on a single sensortube as provided in FIG. 45.

FIG. 47 depicts a second embodiment of a sensor tube in accordance withembodiments of the present disclosure; and

FIG. 48 provides an alternate embodiment of a sensor tube having morethan one sensor tag in accordance with embodiments of the presentdisclosure.

In the drawings, similar elements will be similarly numbered wheneverpossible. However, this practice is simply for convenience of referenceand to avoid unnecessary proliferation of numbers, and is not intendedto imply or suggest that the present disclosure requires identity ineither function or structure in the several embodiments.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention are illustrated in theFIGs., like numerals being used to refer to like and corresponding partsof the various drawings.

Throughout this description, the terms assert and negate may be usedwhen referring to the rendering of a signal, signal flag, status bit, orsimilar apparatus into its logically true or logically false state,respectively, and the term toggle to indicate the logical inversion of asignal from one logical state to the other. Alternatively, reference maybe made to the mutually exclusive Boolean states as logic_0 and logic_1.Of course, as is well known, consistent system operation can be obtainedby reversing the logic sense of all such signals, such that signalsdescribed herein as logically true become logically false and viceversa. Furthermore, it is of no relevance in such systems which specificvoltage levels are selected to represent each of the logic states.

Embodiments of the present invention provide passive radio frequencyidentification (RFID) sensor. This passive RFID sensor includes anantenna, a processing module, and a wireless communication module. Theantenna has an antenna impedance that may vary with an environment inwhich the antenna is placed. The processing module couples to theantenna and has a tuning module that may vary a reactive componentimpedance coupled to the antenna in order to change a system impedance.The system impedance including both the antenna impedance and thereactive component impedance. The tuning module then produces animpedance value representative of the reactive component impedance. Amemory module may store the impedance value which may then later becommunicated to an RFID reader via the wireless communication module.The RFID reader may then exchange the impedance value representative ofthe reactive components of impedance with the RFID reader such that theRFID reader or another external processing unit may process theimpedance value in order to determine environmental conditions at theantenna. These environmental conditions may include but are not limitedto temperature, humidity, wetness, or proximity of the RFID reader tothe passive RFID sensor.

Embodiments of the present disclosure provide a self-tuning passive RFIDsensors that enable a wide variety of applications. One embodimentprovides a sensor for pressure or proximity sensing using a conventionalcompact dipole antenna augmented with a simple floating sheet of metal.The sensor exploits the basic electromagnetic effect where a sheet ofmetal brought in proximity to an inductive loop lowers the inductance ofthe loop due to eddy currents generated on the sheet of metal. Thecloser the sheet gets to the loop, the lower the inductance.

Conventional dipole design for RFID tags use a small inductive loop totune out the input capacitance of the RFID IC. By placing a metal sheetnear this inductive tuning loop, the inductance depends on the distancebetween the loop and the sheet. The self-tuning engine detects thechange in inductance and adjusts its input capacitance to maintain peakpower to the die. The change in capacitance can be read from the die asa sensor code using the standard EPC read command. The sensor codereflects the relative position of the sheet to the antenna inductor.

A proximity sensor mounts the tag onto one surface and a metal patchonto another surface that moves relative to the tag. As the patch movescloser to the tag, the inductance of the tuning loop decreases. Theself-tuning engine compensates for the lower inductance with highercapacitance which is then readable as a sensor code with higher value.The sensor reports closer proximity with higher sensor codes.

The proximity sensor can be converted into a pressure sensor by using apressure sensitive spring between the sheet and the inductor. A simplespring is a small block of closed cell foam, which changes its thicknesswith pressure. Higher pressures compress the foam and bring the metalsheet closer to the inductor, lowering its inductance. Just as for theproximity sensor, the self-tuning engine compensates for the lowerinductance with higher capacitance leading to a larger sensor code. Thesensor reports higher pressure as higher sensor codes.

The proximity/pressure sensor uses a conventional compressed dipole withan inductive tuning loop fabricated on PET, polyimide, or other similarplastic material. Possible metallization patterns are discussed withreference to FIGS. 17H, 20, 26, 30A, 30B, 31 and 33. As will be shown,the sensor incorporates a metal patch about the size of the inductivetuning loop placed directly over the tuning loop. The gap between thepatch and the antenna can range, in one embodiment the gap varies fromabout 0.5 mm to 3 mm. The area enclosed by the inductive tuning loopmust be tuned for the application so that the sensor code stays withinits total tuning range. The sensor code changes as the gap between thetuning loop and the metal patch varies, so the design target would varythe codes within a predetermined range for the smallest gap and thelargest gap, leaving margin for manufacturing and environmentalvariations to avoid pegging the sensor code during normal operation.

For operation as a pressure sensor, the metal patch is mounted over thetuning inductor using closed-cell neoprene foam rubber that can vary inthickness with pressure changes. The sensor codes are averaged toproduce a single average sensor code at each pressure. The sensorachieves very linear response to pressure with low hysteresis. A simplelinear calibration can be applied to convert the average sensor codereading directly to psi. As pressure sensors can use low-cost closedcell foams to implement very low-cost sensors; however, foams can take aset over time or wear out. Higher precision pressure sensors using steelsprings are also possible.

Embodiments of the present disclosure can also serve as a metaldetector, where the presence or absence of metal can be measured. Thepresence of metal in fluid flow in plastic pipe can be measured withpotential applications in food processing.

Proximity applications also include on/off applications, such asopen/closed sensors for doors or windows. As a security seal, the metalcan be stripped off when a container is opened enabling the sensor todetect tampering. Conventional RFID tags can only achieve this functionthrough destruction, leading to the possibility of false positives.

In general, prior disclosures have focused primarily on quantizing thevoltage developed by the tank circuit as the primary means of matchingthe f_(R) of the tank circuit to the transmission frequency f_(C), ofthe received signal. However, this voltage quantization is, at best,indirectly related to received signal field strength. Other effectiveand efficient methods may quantize the received field strength as afunction of induced current. In particular, a method and apparatusadapted to develop this field quantization in a form and manner that issuitable for selectively varying the input impedance of the receivercircuit to maximize received power, especially during normal systemoperation. Additionally, in light of the power sensitive nature of RFIDsystems, our disclosed method and apparatus varied the input impedancewith a minimum power loss.

While prior disclosures use methods to sense environmental changes towhich the RFID tag is exposed. Embodiments of the present disclosurefurther develop this capability and disclose embodiments specificallyadapted to operate in a variety of environments.

Shown in FIG. 3 is an RF receiver circuit 300 suitable for use m an RFIDapplication. As we have described in our Related References, an RFsignal electromagnetically coupled to an antenna 302 is received via atank circuit 304, the response frequency f_(R), of which is dynamicallyvaried by a tuner 306 to better match the transmission frequency, f_(C),of the received RF signal, thus obtaining a maximum power transfer. Inparticular, the RMS voltage induced across the tank circuit 304 by thereceived RF signal is quantized by tuner 306 and the developedquantization employed to control the impedance of the tank circuit 304.Also, the unregulated, AC current induced in the tank circuit by thereceived RF signal is conditioned by a regulator 308 to provideregulated DC operating power to the receiver circuit 300. In accordancewith our present disclosure, we now provide a field strength detector310, also known as a power detector, adapted to develop a field-strengthvalue as a function of the field strength of the received RF signal. Asindicated in FIG. 3, field strength detector 310 is adapted to cooperatewith the regulator 308 in the development of the field-strength value.As disclosed below, if desired, field strength detector 310 can beadapted to cooperate with the tuner 306 in controlling the operatingcharacteristics of the tank circuit 304.

Shown by way of example in FIG. 4 is one possible embodiment of a fieldstrength or power detector 400 (field strength detector 310 of FIG. 3).This embodiment employs a shunt-type regulator 402 so that, duringnormal operation, the shunted ‘excess’ current can be used as areference against which we develop the field-strength value. In thisregard, reference module 404 produces a shunt current reference valueproportional to the shunted current, and then develops a mirroredcurrent reference value as a function of both the shunted current and afield strength reference current provided by a digitally-controlledcurrent source 406. Preferably, once the tuner 306 has completed itsinitial operating sequence, whereby the f_(R) of the tank circuit 304has been substantially matched to the f_(C) of the received signal, wethen enable a digital control 408 to initiate operation of the currentsource 406 at a predetermined, digitally established minimum fieldstrength reference current. After a predetermined period of time,control 408 captures the mirrored current reference value provided bythe current reference module 404, compares the captured signal against apredetermined threshold value, and, if the comparison indicates that thefield strength reference current is insufficient, increases, inaccordance with a predetermined sequence of digital-controlledincrements, the field strength reference current; upon the comparisonindicating that the field strength reference current is sufficient,control 408 will, at least temporarily, cease operation.

In accordance with embodiments of the present disclosure, the digitalfield-strength value developed by control 408 to control the fieldstrength current source 406 is a function of the current induced in thetank circuit 304 by the received RF signal. Once developed, this digitalfield-strength value can be employed in various ways. For example, itcan be selectively transmitted by the RFID device (using conventionalmeans) back to the reader (not shown) for reference purposes. Such atransaction can be either on-demand or periodic depending on systemrequirements. Imagine for a moment an application wherein a plurality ofRFID tag devices are distributed, perhaps randomly, throughout arestricted, 3-dimensional space, e.g., a loaded pallet. Imagine alsothat the reader is programmed to query, at an initial field strength,all tags “in bulk” and to command all tags that have developed afield-strength value greater than a respective field-strength value toremain ‘silent’. By performing a sequence of such operations, each at anincreasing field strength, the reader will, ultimately, be able toisolate and distinguish those tags most deeply embedded within thespace; once these ‘core’ tags have been read, a reverse sequence can beperformed to isolate and distinguish all tags within respective,concentric ‘shells’ comprising the space of interest. Although, in alllikelihood, these shells will not be regular in either shape or relativevolume, the analogy should still be apt.

In FIG. 5, one possible embodiment of a field strength detector 500 isillustrated. In general, shunt circuit 502 to develop a substantiallyconstant operating voltage level across supply node 504 and ground node506. Shunt regulators of this type are well known in the art, andtypically use Zener diodes, avalanche breakdown diodes, diode-connectedMOS devices, and the like.

As can be seen, current reference 404 of FIG. 4 is implemented in theform of a current mirror circuit 508, connected in series with shuntcircuit 502 between nodes 504 and 506. As is typical, current mirrorcircuit 508 comprises a diode-connected reference transistor 510 and amirror transistor 512. If desired, a more sophisticated circuit such asa Widlar current source may be used rather than this basictwo-transistor configuration. For convenience of reference, the currentshunted by shunt circuit 502 via reference transistor 510 is designatedas i_(R); similarly, the current flowing through mirror transistor 512is designated i_(R) as i_(R)/N, wherein, as is known, N is the ratio ofthe widths of reference transistor 510 and mirror transistor 512.

Here, the field strength current source 516 is implemented as a set of nindividual current sources, each connected in parallel between thesupply node 504 and the mirror transistor 512. In general, fieldstrength current source 516 is adapted to source current at a levelcorresponding to an n-bit digital control value developed by a counter518. In the illustrated embodiment wherein n=5, field strength currentsource 516 is potentially capable of sourcing thirty-two distinctreference current levels. We propose that the initial, minimum referencecurrent level be selected so as to be less than the current carryingcapacity of the mirror transistor 512 when the shunt circuit 502 firstbegins to shunt excess induced current through reference transistor 510;that the maximum reference current level be selected so as to be greaterthan the current carrying capacity of the mirror transistor 512 when theshunt circuit 502 is shunting a maximum anticipated amount of excessinduced current; and that the intermediate reference current levels bedistributed relatively evenly between the minimum and maximum levels. Ofcourse, alternate schemes may be practicable, and, perhaps, desirabledepending on system requirements.

Within control 518, a conventional analog-to-digital converter (“ADC”)520, having its input connected to a sensing node 514, provides adigital output indicative of the field strength reference voltage,v_(R), developed on sensing node 514. In one embodiment, ADC 520 maycomprise a comparator circuit adapted to switch from a logic_0 state toa logic_1 when sufficient current is sourced by field strength currentsource 516 to raise the voltage on sensing node 514 above apredetermined reference voltage threshold, v_(th). Alternatively, ADC520 may be implemented as a multi-bit ADC capable of providing higherprecision regarding the specific voltage developed on sensing node 514,depending on the requirements of the system. Sufficient current may becharacterized as that current sourced by the field strength currentsource 516 or sunk by mirror transistor 512 such that the voltage onsensing node 514 is altered substantially above or below a predeterminedreference voltage threshold, v_(th). In the exemplary case of a simpleCMOS inverter, v_(th) is, in its simplest form, one-half of the supplyvoltage (VDD/2). Those skilled in the art will appreciate that v_(th)may by appropriately modified by altering the widths and lengths of thedevices of which the inverter is comprised. In the exemplary case amulti-bit ADC, v_(th) may be established by design depending on thesystem requirements and furthermore, may be programmable by the system.

In the illustrated embodiment, a latch 522 captures the output state ofADC 520 in response to control signals provided by a clock/controlcircuit 524. If the captured state is logic_0, the clock/control circuit524 will change counter 518 to change the reference current beingsourced by field strength current source 516; otherwise clock/controlcircuit 524 will, at least temporarily, cease operation. However,notwithstanding, the digital field-strength value developed by counter518 is available for any appropriate use, as discussed above.

By way of example, we have illustrated in FIG. 6 one possible generaloperational flow of a field strength detector in accordance withembodiments of the present disclosure. Upon activation, counter 518 isset to its initial digital field-strength value (step 602), therebyenabling field strength current source 516 to initiate reference currentsourcing at the selected level. After an appropriate settling time, thefield strength reference voltage, v_(R), developed on sensing node 514and digitized by ADC 520 is captured in latch 522 (step 604). If thecaptured field strength reference voltage, v_(R), is less than (or equalto) the predetermined reference threshold voltage, v_(th), clock/control524 will change counter 518 (step 606). This process will repeat,changing the reference current sourced by field strength current source516 until the captured field strength reference voltage, v_(R), isgreater than the predetermined reference threshold voltage, v_(th), (atstep 608), at which time the process will stop (step 610). Asillustrated, this sweep process can be selectively reactivated asrequired, beginning each time at either the initial field-strength valueor some other selected value within the possible range of values asdesired.

The graph illustrated in FIG. 7 depicts several plots of the voltagedeveloped on sensing node 514 as the field strength detector circuit 400sweeps the value of counter 518 according to the flow illustrated inFIG. 6. As an example, note that the curve labeled “A” in FIG. 5 beginsat a logic_0 value when the value of counter 38 is at a minimum valuesuch as “1” as an exemplary value. Subsequent loops through the sweeploop gradually increase the field strength reference voltage on sensingnode 514 until counter 38 reaches a value of “4” as an example. At thispoint, the “A” plot in FIG. 5 switches from a logic_0 value to a logic_1value, indicating that the field strength reference voltage, v_(R), onsensing node 514 has exceeded the predetermined reference thresholdvoltage, v_(th). Other curves labeled “B” through “D” depict incrementalincreases of reference currents, i_(R), flowing through reference device510, resulting in correspondingly higher mirrored currents flowingthrough mirror device 512. This incrementally higher mirror currentrequires field strength current source 406 to source a higher currentlevel which in turn corresponds to higher values in counter 518. Thus,it is clear that embodiments of the present disclosure are adapted toeffectively and efficiently develop a digital representation of thecurrent flowing through sensing node 514 that is suitable for anyappropriate use.

One such use, as discussed earlier, of our field strength detector 310is to cooperate with tuner 306 in controlling the operatingcharacteristics of the tank circuit 304. FIG. 8 illustrates one possibleembodiment where receiver circuit 800 uses a field strength detector 802specially adapted to share with tuner 804 the control of the tankcircuit 806. Dynamically tuning, via tuner 306 a, the tank circuit 806allows one to dynamically shift the/R of the tank circuit 806 to bettermatch the f_(C) of the received RF signal at antenna 808. FIG. 8 adds amultiplexer 810 to tuner 804 to facilitate shared access to the tunercontrol apparatus. Shown in FIG. 9 is the operational flow of fieldstrength detector 800 upon assuming control of tank circuit 806.

In context of this particular use, once tuner 804 has completed itsinitial operating sequences, and field strength detector 500 hasperformed an initial sweep (as described above and illustrated in FIG.6) and saved in a differentiator 812 a base-line field-strength valuedeveloped in counter 814, clock/control 816 commands multiplexer 810 totransfer control of the tank circuit 806 to field strength detector 802(all comprising step 902 in FIG. 9). Upon completing a second currentsweep, differentiator 812 will save the then-current field-strengthvalue developed in the counter 814 (step 904). Thereafter,differentiator 812 will determine the polarity of the change of thepreviously saved field-strength value with respect to the then-currentfield-strength value developed in counter 814 (step 906). If thepolarity is negative (step 908), indicating that the currentfield-strength value is lower than the previously-saved field strengthvalue, differentiator 812 will assert a change direction signal;otherwise, differentiator 812 will negate the change direction signal(step 910). In response, the shared components in tuner 804 downstreamof the multiplexer 810 will change the tuning characteristics of tankcircuit 806 (step 912). Now, looping back (to step 904), the resultingchange of field strength, as quantized is the digital field-strengthvalue developed in counter 814 during the next sweep (step 904), will bedetected and, if higher, will result in a further shift in the f_(R) ofthe tank circuit 806 in the selected direction or, if lower, will resultin a change of direction (step 910). Accordingly, over a number of such‘seek’ cycles, embodiments of the present disclosure will selectivelyallow the receiver 800 to maximize received field strength even if, as aresult of unusual factors, the f_(R) of the tank circuit 806 may not beprecisely matched to the f_(C) of the received RF signal, i.e., thereactance of the antenna is closely matched with the reactance of thetank circuit, thus achieving maximum power transfer. In an alternativeembodiment, it would be unnecessary for tuner 804 to perform an initialoperating sequence. Rather, field strength detector 802 may be usedexclusively to perform both the initial tuning of the receiver circuit800 as well as the subsequent field strength detection. Note that thesource impedance of antenna 808 and load impedance of tank circuit 806may be represented alternatively in schematic form as in FIG. 10,wherein antenna 808 is represented as equivalent source resistance R_(S)1002 and equivalent source reactance X_(S) 1004, and tank circuit 806 isrepresented as equivalent load resistance R_(L) 1006 and equivalent,variable load reactance X_(L) 1008.

FIG. 11 illustrates alternate embodiments of a field strength detector1100, previously discussed with reference to FIG. 5. Here, as before,shunt circuit 502 is used to develop a substantially constant operatingvoltage level across supply node 504 and ground node 506. Also asbefore, the current reference 516 is implemented as a current mirrorcircuit 508 connected in series with shunt circuit 502 between nodes 504and 506. However, in this embodiment, the field strength current sourcecomprises a resistive component 1102 adapted to function as a staticresistive pull-up device. Many possible implementations exist besides abasic resistor, such as a long channel length transistor, and thoseskilled in the art will appreciate the various implementations that areavailable to accomplish analogous functionality. The field strengthvoltage reference v_(R) developed on sensing node 514 will be drawn to astate near the supply voltage when the mirrored current flowing throughtransistor 512 is relatively small, e.g. close to zero amps, indicatinga weak field strength. As the field strength increases, the currentflowing through mirror transistor 512 will increase, and the fieldstrength voltage reference VR developed on sensing node 514 will dropproportionally to the mirrored current flowing through mirror transistor512 as i_(R)/N. ADC 40, having its input connected to sensing node 514,provides a digital output indicative of the field strength referencevoltage, v_(R), developed on sensing node 514, as described previously.

In this alternate embodiment, latch 42 captures the output state of ADC40 in response to control signals provided by a clock/control circuit524. As disclosed earlier, the ADC 40 may comprise a comparator circuit.In this instance, ADC 40 is adapted to switch from a logic 1 state to alogic_0 when sufficient current is sunk by mirror transistor 512 tolower the voltage on sensing node 514 below a predetermined referencevoltage threshold, v_(th). Alternatively, ADC 40 may be implemented as amulti-bit ADC capable of providing higher precision regarding thespecific voltage developed on sensing node 514, depending on therequirements of the system.

Comparator 1104 subsequently compares the captured output state held inlatch 520 with a value held in counter 518 that is selectivelycontrolled by clock/control circuit 524. In response to the outputgenerated by comparator 1104, clock/control circuit 524 may selectivelychange the value held in counter 518 to be one of a higher value or alower value, depending on the algorithm employed. Depending upon theimplementation of counter 518 and comparator 1104, clock/control circuit524 may also selectively reset the value of counter 518 or comparator1104 or both. The digital field-strength value developed by counter 518is available for any appropriate use, as discussed above.

In FIG. 12 we have illustrated another alternate embodiment of our fieldstrength detector 1200 illustrated in FIG. 5. Here, as before, shuntcircuit 502 is used to develop a substantially constant operatingvoltage level across supply node 504 and ground node 506. In thisembodiment, the current reference is implemented as a resistivecomponent 1202 that functions as a static pull-down device. Manypossible implementations exist besides a basic resistor, such as a longchannel length transistor and those skilled in the art will appreciatethe various implementations that are available to accomplish analogousfunctionality. The field strength voltage reference v_(R) developed onsensing node 514 will be drawn to a state near the ground node when thecurrent flowing though shunt circuit 502 is relatively small, e.g. closeto zero amps, indicating a weak field strength. As the field strengthincrease, the current flowing through shunt circuit 502 will increase,and the field strength voltage reference VR developed on sensing node514 will rise proportionally to the current flowing through shuntcircuit 502. ADC 520, having its input connected to a sensing node 514,provides a digital output indicative of the field strength referencevoltage, v_(R), developed on sensing node 514, as described previously.

In this alternate embodiment, latch 522 captures the output state of ADC520 in response to control signals provided by a clock/control circuit524. As disclosed earlier, the ADC 520 may comprise a comparatorcircuit. In this instance, ADC 520 is adapted to switch from a logic_0state to a logic_1 when sufficient current is sourced by shunt circuit502 to raise the voltage on sensing node 514 above a predeterminedreference voltage threshold, v_(th). Alternatively, ADC 520 may beimplemented as a multi-bit ADC capable of providing higher precisionregarding the specific voltage developed on sensing node 514, dependingon the requirements of the system.

Comparator 1104 subsequently compares the captured output state held inlatch 522 with a value held in counter 518 that is selectivelycontrolled by clock/control circuit 524. In response to the outputgenerated by comparator 1104, clock/control circuit 524 may selectivelychange the value held in counter 518 to be one of a higher value or alower value, depending on the algorithm employed. Depending upon theimplementation of counter 518 and comparator 1104, clock/control circuit524 may also selectively reset the value of counter 518 or comparator1104 or both. The digital field strength value developed by counter 518is available for any appropriate use, as discussed above.

In another embodiment, embodiments of the present disclosure may beadapted to sense the environment to which a tag is exposed, as well assensing changes to that same environment. The auto-tuning capability oftuner 306 acting in conjunction with tank circuit 304 detects antennaimpedance changes. These impedance changes may be a function ofenvironmental factors such as proximity to interfering substances, e.g.,metals or liquids, as well as a function of a reader or receiver antennaorientation. Likewise, as disclosed herein, field strength (i.e.,received power) detector 310 may be used to detect changes in receivedpower (i.e., field strength) as a function of, for example, poweremitted by the reader, distance between tag and reader, physicalcharacteristics of materials or elements in the immediate vicinity ofthe tag and reader, or the like. Sensing the environment or, at least,changes to the environment is accomplished using one or both of thesecapabilities.

As an example, the tag 1300 of FIG. 13, contains both a source tagantenna (not shown, but see, e.g., FIG. 8) and a corresponding load chiptank circuit 304 (not shown, but see, e.g., FIG. 8). Each contains bothresistive and reactive elements as discussed previously (see, e.g., FIG.10). Tag 1300 containing such a tank circuit 304 mounted on a metallicsurface will exhibit antenna impedance that is dramatically differentthan the same tag 1300 in free space or mounted on a container ofliquid. Shown in Table 1 are exemplary values for impedance variationsin both antenna source resistance 1002 as well as antenna sourcereactance 1004 as a function of frequency as well as environmentaleffects at an exemplary frequency.

TABLE 1 Antenna Impedance Variations 860 MHz 870 MHz 880 MHz 890 MHz Rs,Xs, Rs, Xs, Rs, Xs, Rs, Xs, In Air 1.3 10.7 1.4 10.9 1.5 11.2 1.6 11.5On Metal 1.4 10.0 1.5 10.3 1.6 10.6 1.7 10.9 On Water 4.9 11.3 1.8 11.12.4 11.7 2.9 11.5 On Glass 1.8 11.1 2.0 11.4 2.2 11.7 2.5 12.0 OnAcrylic 1.4 10.6 1.6 11.1 1.7 11.4 1.9 11.7 900 MHz 910 MHz 920 MHz 930MHz Rs, Xs, Rs, Xs, Rs, Xs, Rs, Xs, In Air 1.8 11.8 2.0 12.1 2.2 12.42.4 12.8 On Metal 1.9 11.2 2.1 11.6 2.3 12.0 2.6 12.4 On Water 2.5 12.33.0 12.7 5.8 14.1 9.1 13.2 On Glass 2.8 12.4 3.2 12.8 3.7 13.2 4.2 13.6On Acrylic 2.0 12.1 2.3 12.4 2.5 12.8 2.8 13.2

The tuner circuit 306 of embodiments of the present disclosureautomatically adjusts the load impendence by adjusting load reactance1008 (see, e.g., FIG. 10

FIG. 8) to match source antenna impedance represented by sourceresistance 1002 (see, e.g., FIG. 10) and source reactance 1004 (see,e.g., FIG. 10). As previously disclosed, matching of the chip loadimpedance and antenna source impedance can be performed automatically inorder to achieve maximum power transfer between the antenna and thechip. A digital shift register 1302 allows selectively changing thevalue of the load reactive component 1008 (see, e.g., FIG. 10), in thepresent case a variable capacitor, until power transfer is maximized.This digital value of the matched impendence may be used eitherinternally by the tag 1300, or read and used by the reader 1304, todiscern relative environmental information to which the tag 1300 isexposed. For example, tag 1300 may contain a calibrated look-up-tablewithin the clock/control circuit 524 which may be accessed to determinethe relevant environmental information. Likewise, a RFID reader 1304 mayissue commands (see transaction 1 in FIG. 13) to retrieve (seetransaction 2 in FIG. 13) the values contained in digital shift register1302 via conventional means, and use that retrieved information toevaluate the environment to which tag 1300 is exposed. The evaluationcould be as simple as referencing fixed data in memory that has alreadybeen stored and calibrated, or as complex as a software applicationrunning on the reader or its connected systems for performinginterpretive evaluations.

Likewise, consider a tag 1300 containing our field strength (i.e.,received power) detector 310 (not shown, but, e.g., see FIG. 3) whereinthe method of operation of the system containing the tag 1300 calls forfield strength detector 310 to selectively perform a sweep function anddeveloping the quantized digital representation of the current via themethod discussed earlier. As illustrated in FIG. 13, counter 518 willcontain the digital representation developed by our field strengthdetector 310 of the RF signal induced current, and may be used eitherinternally by the tag 1300, or read and used by the reader 1304, todiscern relative environmental information to which the tag 1300 isexposed. For example, reader 1304 may issue a command to the tag 1300(see transaction 1 in FIG. 13) to activate tuner 306 and/or detector 310and, subsequent to the respective operations of tuner 306 and/ordetector 310, receive (see transaction 2 in FIG. 13) the digitalrepresentations of either the matched impedance or the maximum currentdeveloped during those operations. Once again, this digital value of thefield strength stored in the counter 518 may be used either internallyby the tag 1300, or read and used by the reader 1304, to discernrelative environmental information to which the tag 1300 is exposed. Forexample, tag 1300 may contain a calibrated look-up-table within theclock and control block 524 which may be accessed to determine therelevant environmental information. Likewise, a RFID reader may issuecommands to retrieve the values contained in digital shift register1302, and use that retrieved information to evaluate the environment towhich tag 1300 is exposed. The evaluation could be as simple asreferencing fixed data in memory that has already been stored andcalibrated, or as complex as a software application running on thereader or its connected systems for performing interpretive evaluations.Thus, the combining of the technologies enables a user to sense theenvironment to which a tag 1300 is exposed as well as sense changes tothat same environment.

Some environmental factors can change the effective impedance of theRFID antenna. Thus, it is possible to dynamically retune the tankcircuit 304 or other like impedance to compensate for theenvironmentally-induced change in impedance by systematically changingthe digital tuning parameters of tank circuit 304. By characterizing theantenna impedance as a function of various factors, one can develop anestimate of the relative change in the environmental factor as afunction of the relative change in the digital tuning parameters of thetank circuit 304.

As can be seen in Table 1, above, it is possible to develop, a priori, areference table storing information relating to a plurality ofenvironmental reference conditions. Thereafter, in carefully controlledconditions wherein one and only one environmental condition of interestis varied (see, FIG. 14), an operational tag 1300 is exposed to each ofthe stored reference conditions (step 1402) and allowed to complete thetank tuning process. (recursive steps 1406 and 1408. After tuning hasstabilized, the tag 1300 can be interrogated (step 1410), and the finalvalue in the shift register 1302 retrieved (step 1410). This value isthen stored in the reference table in association with the respectiveenvironmental condition (step 1412). The resulting table might look likethis:

TABLE 2 Tuning Parameters vs Frequency 860 870 880 890 900 910 920 930MHz MHz MHz MHz MHz MHz MHz MHz InAir 25 21 16 12  8 4  0  0* On Metal31 27 22 17 12 8  3  0  On Water 20 19 12 12   4 0  0* 0* On Glass 21 1712  8 4 0* 0* 0* On Acrylic 23 19 14 10  6 2  0* 0* 0* indicates that alower code was needed but not available, 0 is a valid code

In contrast to prior art systems in which the antenna impedance must beestimated indirectly, e.g., using the relative strength of the analogsignal returned by a prior art tag 1300 in response to interrogation bythe reader 1304, methods of the present disclosure employ the on-chipre-tuning capability of our tag 1300 to return a digital value whichmore directly indicates the effective antenna impedance. Using areference table having a sufficiently fine resolution, it is possible todetect even modest changes in the relevant environmental conditions. Itwill be readily realized by practitioners in this art that, in generalapplications, environment conditions typically do not change in an idealmanner, and, more typically, changes in one condition are typicallyaccompanied by changes in at least one other condition. Thus, antennadesign will be important depending on the application of interest.

One possible approach mounts the antenna on a substrate that tends toamplify the environmental condition of interest, e.g., temperature.

Shown in FIGS. 15A and 15B is an RF sensing system 1500 constructed inaccordance with one embodiment of embodiments of the present disclosure,and specially adapted to facilitate sensing of one or more environmentalconditions in a selected region surrounding the system 1500. In general,the system 1500 comprises: an RF transceiver 1506; a di-pole antenna1508 comprising a pole 1508 a and an anti-pole 1508 b; and a tail 1510of effective length T, comprising respective transmission line pole 1510a and transmission line anti-pole 1510 b, each of length T/2. Inaccordance with embodiments of the present disclosure, the differentialtransmission line elements 1510 a-1510 b are symmetrically coupled torespective poles 1508 a-1508 b at a distance d from the axis of symmetryof the antenna 1508 (illustrated as a dotted line extending generallyvertically from the transceiver 1506). In general, d determines thestrength of the interaction between the transmission line 1510 and theantenna 1508, e.g., increasing d tends to strengthen the interaction. Inthe equivalent circuit shown in FIG. 15B, the voltage differentialbetween the complementary voltage sources 1508 a and 1508 b tends toincrease as d is increased, and to decrease as d is decreased.Preferably d is optimized for a given application. However, it will berecognized that the sensitivity of the antenna may be degraded as afunction of d if a load, either resistive or capacitive, is imposed onthe tail 1510.

In operation, the tail 1510 uses the transmission line poles 1510 a-1510b to move the impedance at the tip of the tail 1510 to the antenna 1508,thus directly affecting the impedance of the antenna 1508. Preferably,the transceiver 1506 incorporates our tuning circuit 306 so as to detectany resulting change in antenna impedance and to quantize that changefor recovery, e.g., using the method we have described above withreference to FIG. 14.

FIG. 16 illustrates one possible embodiment of the system 1600 in whichthe antenna poles 1508 a-1508 b are instantiated as a patch antenna(illustrated in light grey), with the antenna pole 1508 a connected toone output of transceiver 1506, and the other output of transceiver 1506connected to the antenna anti-pole 1508 b. A ground plane 1512 a(illustrated in a darker shade of grey than the patch antenna 1508) isdisposed substantially parallel to both the antenna poles 1508 a-1508 band a ground plane 1512 b disposed substantially parallel to thetransmission line poles 1510 a-1510 b. As is known, the ground planes1512 are separated from the poles by a dielectric substrate (not shown),e.g., conventional flex material or the like. If the dielectric layerbetween the antenna poles 1508 and ground plane 1512 a is of a differentthickness than the layer between the transmission line poles 1510 andthe ground plane 1512 b, the ground plane 1512 b may be disconnectedfrom the ground plane 1512 a and allowed to float. In general, thisembodiment operates on the same principles as described above withreference to FIGS. 15A and 15B.

Shown in FIGS. 17 A-H is an antenna constructed in accordance with oneother embodiment of embodiments of the present disclosure, and speciallyadapted for use in the sensing system to facilitate sensing the presenceof fluids; and, in particular, to the depth of such fluids. In theillustrated embodiment, antenna 1700 comprises a head portion 1506 and atail portion 1508. In general, the head 1506 is adapted to receive RFsignals and to transmit responses using conventional backscattertechniques; whereas the tail portion 1508 functions as a transmissionline. During normal operation, the tail 1508 acts to move and transformthe impedance at the tip of the tail to the head 1506. Accordingly, anychange in the tip impedance due to the presence of fluid willautomatically induce a concomitant change in the impedance of the headantenna 1506. As has been explained above, a tuning circuit will detectthat change and re-adjust itself so as to maintain a reactive impedancematch. As has been noted above, any such adjustment is reflected inchanges in the digital value stored in shift register 1302 (FIG. 13).

Shown in FIG. 18 is one possible flow for a sensing system 1500 usingthe antenna 114. As has been explained above with reference to FIG. 14,operations 1800 begins with the sensor being first calibrated (step 1802to detect the presence of varying levels of a particular substance. Forthe purposes of this discussion, we mean the term substance to mean anyphysical material, whether liquid, particulate or solid, that is:detectable by the sensor; and to which the sensor demonstrably responds.By detectable, we mean that, with respect to the resonant frequency ofthe antenna in the absence of the substance, the presence of thesubstance in at least some non-trivial amount results in a shift in theresonant frequency of the antenna, thereby resulting in a concomitantadjustment in the value stored in the shift register 1302; and bydemonstrably responds we mean that the value stored in the shiftregister 1302 varies as a function of the level the substance relativeto the tip of the tail 1506 of the antenna 1700. Once calibrated, thesensor can be installed in a structure (step 1804), wherein thestructure can be open, closed or any condition in between. The structurecan then be exposed to the substance (step 1806), wherein the means ofexposure can be any form appropriate for both the structure and thesubstance, e.g., sprayed in aerosol, foam or dust form, immersed inwhole or in part in a liquid, or other known forms. Following a periodof time deemed appropriate for the form of exposure, the sensor isinterrogated (step 1808) and the then-current value stored in the shiftregister 1302 retrieved. By correlating this value with the table ofcalibration data gathered in step 1802, the presence or absence of thesubstance can be detected (step 1810).

In one embodiment, the table of calibration data can be stored in thesensor and selectively provided to the reader during interrogation toretrieve the current value. Alternatively, the table can be stored in,e.g., the reader and selectively accessed once the current value hasbeen retrieved. As will be clear, other embodiments are possible,including storing the table in a separate computing facility adapted toselectively perform the detection lookup when a new current value hasbeen retrieved.

Assume by way of example, an automobile assembly line that includes asan essential step the exposure, at least in part, of apartially-assembled automobile chassis to strong streams of a fluid,e.g., water, so as to determine the fluid-tightness of the chassis.Given the complexity of a modern automobile, it is not cost effective tomanually ascertain the intrusion of the fluid at even a relatively smallnumber of possible points of leakage. However, using our sensors andsensing system, it is now possible to install relatively large numbersof independently-operable sensors during the assembly process, even inhighly inaccessible locations such as largely-enclosed wiring channelsand the like. In the course of such installations, the unique identitycodes assigned to each installed sensor is recorded together withpertinent installation location details. After extraction from theimmersion tank, the chassis can be moved along a conventional conveyorpath past an RFID reader sited in a position selected to facilitateeffective querying of all of the installed sensors. In one embodiment,the reader may be placed above the moving chassis so as to “look down”through the opening provided for the front windshield (which may or maynot be installed) into the interior portion of the chassis; from such aposition even those sensors installed in the “nooks and crannies” in thetrunk cavity should be readable. By correlating the code read from eachsensor with the previously constructed, corresponding table, it is nowpossible to detect the presence (or absence) of the substance at therespective location of that sensor; indeed, if the sensor issufficiently sensitive to the substance, it may be possible to estimatethe severity of the leakage in the vicinity of each sensor.

Shown in FIG. 19 is an antenna 1900 constructed in accordance with anembodiment of the present disclosure, and specially adapted for use inthe sensing system 1500 to facilitate sensing the presence of fluids;and, in particular, to the depth of such fluids. As illustrated in FIG.19A, the top layer of antenna 1900 comprises: a patch antenna portion1906; an antenna ground plane 1904; a tail portion 1908; and a dieattach area 1910. As noted in FIG. 19A, the tail portion 1908 comprisesa pair of generally parallel transmission lines each substantially thesame in length. As illustrated in FIG. 19B, the bottom layer of antenna1900 comprises a ground plane for the transmission lines 1912. During atypical assembly process, the illustrated shapes are formed in the topand bottom layers of a continuous roll of copper-clad flex circuitmaterial, and each antenna 1900 cut from the roll using a rolling cutterassembly. An RFID tag device (incorporating a tuning circuit) is thenattached to the die attach area 1910, and the antenna 1900 is foldedalong fold lines 1 and 2 generally around a suitable core material suchas PET or either open-cell or closed-cell foam.

In general, the patch antenna portion 1906 is adapted to receive RFsignals and to transmit responses using conventional backscattertechniques. During normal operation, the transmission lines 1908comprising the tail 1902 act to move and transform the impedance at thetip of the tail 1902 to the patch antenna 1906. Accordingly, any changein the tip impedance due to the presence of fluid will automaticallyinduce a concomitant change in the impedance of the head antennal. Ashas been explained above, our tuning circuit 306 will detect that changeand re-adjust itself so as to maintain a reactive impedance match. Ashas been noted above, any such adjustment is reflected in changes in thedigital value stored in shift register 1302 (FIG. 13).

FIG. 20 is depiction of an antenna inlay that may be used in accordancewith embodiments of the present disclosure.

FIG. 21 is a block diagram of a RFID wireless solution provided byembodiments of the present disclosure. Integrated circuit (IC) 2100comprises a memory module 2102, a wireless communication engine 2104,and a sensor engine 2106 which includes an antenna 2108. IC 2100 iscapable of sensing a change in the environmental perimeters proximate toIC 2100 via impedance changes associated with antenna 2108. In otherembodiments a proximity sensor may be employed to determine theproximity of IC 2100 to a given location or RFID reader by tuning theantenna 2108 and an associated tunable impedance. Memory module 2102 iscoupled with both the wireless communication engine 2104 and sensorengine 2106. Memory module 2102 is capable of storing information anddata gathered by sensor engine 2106 and communicated via wirelesscommunication engine 2104. Further, wireless communication engine 2104and sensor engine 2106 may be fully programmable via wireless methods.Passive RFID sensors of FIG. 21 may be deployed as an array of smartsensors or agents to collect data that may be sent back to a centralprocessing unit.

FIG. 22 is a block diagram of one arrangement of smart sensors and adata processing unit 2202 in accordance with embodiments of the presentdisclosure. Here a series of passive RFID sensors 2100A-N are deployedwherein each sensor may have a unique identification number storedwithin the memory module and communicated via the internal wirelesscommunications engine 2104 to a data processing unit. Interrogator (RFIDreader) 2204 interacts with passive RFID sensors 2100A-N. Interrogator2204 may then communicate with a data processing unit 2202. Thus thepassive RFID sensor array 2206 may allow information to be sensed andcommunicated via RFID reader 2204, wherein this information may bepreprocessed at the passive RFID sensor, or remotely processed at theRFID reader 2204 or data processing unit 2202 depending on the systemneeds.

Embodiments of the present disclosure realize an advantage over priorsystems, in that not all sensing requires high precision sensors whichare both expensive and consume relatively large amounts of power. Thesensors provided by embodiments of the present disclosure are relativemeasurements and post processing of collected measurements yields senseinformation. Calibration may be done during manufacturing at the waferor die level or when the assembled sensors are deployed in the fieldwherein this calibration information may be stored in the memory module2102. This information may be retrieved at any time for baselinecalculations. From relative changes accurate information may then bederived from remote data processing provided by data processing unit2202. Calibration may involve retrieving sensing measurements frommemory module 2102 or current measurements directly form sensor engine2106. The use of this information then allows accurate data associatedwith environmental conditions to be determined. In one example, RFIDsensor array 2206 of FIG. 22 may include temperature sensors. Whereineach passive RFID sensor 2100A-N is an independent sensor and may sensea current condition at time zero that is stored to memory module 2102 orsent to data processing unit 2202. This measurement may be repeated atTime 1. Wherein this data is either stored or transmitted. Dataprocessing unit 2202 may perform more complex calculations. For example,if the temperature is known at Time 0, the sensor information collectedat Time 1, when communicated may be processed using informationassociated with the measurements and known temperature at Time 0 inorder to determine or approximate an actual temperature. This mayinvolve a lookup in a characterized data table or computations based onmathematical models of the calibration of the sensors to determine orapproximate the actual temperature.

Another embodiment can sense the level of wetness or humidity proximateto the sensor engine. In either case, temperature or moisture, raw datamay be collected from passive RFID sensors via the RFID reader forprocessing to be performed by data processing unit 2202 where thecomputation to determine a humidity or temperature measurement.

FIGS. 23A and 23B are views of an RFID seal tag with an antenna sealedby an induction seal in accordance with an embodiment of the presentdisclosure. FIG. 23A provides an isometric depiction of an RFID taghaving a seal tag about the perimeter of the antenna. FIG. 23A shows IC2100 coupled to an antenna 2304. Spacer 2306 may separate antenna 2304from an induction seal 2308. Perimeter ring 2310 about antenna 2304 maybe welded to couple antenna 2304 to induction seal 2306. This is shownisometrically in FIG. 23A, where the antenna 2304 couples to inductionseal 2306 creating a larger antenna surface. This embodiment of apassive RFID sensor tag coupled to the induction seal may be placedwithin a container such as a pill bottle in order to measure temperatureand wetness or humidity inside the pill bottle.

FIGS. 24A and 24B depict different antenna inlays that may be used inaccordance with embodiments of the present disclosure. In FIG. 24A theantenna wing 2402 and wing 2404 may both couple to the induction seal2310. In this case the antenna wings form a loop where the entire edgeof the antenna inlay is attached to the induction seal and the inductionseal completes the loop coupling wings 2402 and 2404. In the embodimentprovided in FIG. 24B, antenna wing 2402 and wing 2404 are not coupled bythe induction seal 2310. In FIG. 24B Left wing 2402 and right wing 2404are separated by IC 2100 wherein left wing 2402 is isolated by a gapfrom the right wing and the induction seal 2308 serves as the right wing2404. Performance is sensitive to the thickness of the tape used toattach the passive RFID sensor. Wherein the passive RFID sensor (pilltag) of FIGS. 23A, 23B, 24A, and 24B may involve an antenna being asingle layer of copper on captain folded and laminated about a PET coreused as a spacer to create a dielectric layer between the antenna andthe induction seal. The RFID tag provided by FIGS. 23A, 23B, 24A, and24B provides an RFID tag that may be attached to a metalized lid insertfor use in a pill bottle or other like use.

FIG. 25 provides an illustration of an antenna arrangement in accordancewith embodiments of the present disclosure. In this antenna arrangement2500 the antenna comprises a first antenna wing 2502 and a secondantenna wing 2504 coupled to IC 2100 Via connections 2506. IC 2100 mayoptimize the impedance match between the IC 2100 and antenna 2500. Thiscan be accomplished by adding shunt capacitors, variable inductors orvariable impedances across the input terminals of IC 2100. As a result,the input impedance of integrated circuit can be varied between in oneembodiment can be varied between 2.4 minus J 67.6 to 0.92 minus J 41.5ohms. An antenna such as that provided in FIG. 25 may be designed tooperate within these impedance values.

In one embodiment this may provide an RF sensitivity of approximately−10.5 DbM. The antenna provided in FIG. 25 may be optimized to provide aconjugate match in one embodiment at about 960 megahertz. This allowsthe integrated circuit to optimize and match by selecting the best MMSvalue over the remaining portion of the frequency band. The operationalbandwidth is proportional to the tag thickness.

FIG. 26 provides an illustration of an antenna arrangement in accordancewith embodiments of the present disclosure. In this antenna arrangement2600 the antenna comprises a first antenna wing 2602 and a secondantenna wing 2604 coupled to IC 2100 Via connections 2606. IC 2100 mayoptimize the impedance match between the IC 2100 and antenna 2600. Firstantenna wing 2602 and second antenna wing 2604 can have interdigitatedportions 2608, wherein the coupling of these interdigitated portions2608 varies with dielectric changes in the gap 2610 between theinterdigitated portions 2608. This can be accomplished by addingvariable impedances across the input terminals of IC 2100.

FIGS. 27A and 27B are graphs of the conjugate match factor (CMF) wherethe passive RFID sensor tags have a thickness of 0.03 and 0.05 inchesthick tags with an MMS value of 0. FIGS. 28A and 28B provide a graph ofthe antenna impedance for the previously depicted antenna of FIG. 25.The rate of change in the impedance data for the thicker version isshown to be less than the rate of change in the impedance for thethinner version. This equates to a larger operation bandwidth. FIGS. 29Aand 29B provide sensitivity graphs for varying thicknesses associatedwith the antenna's impendence, directivity, and radiation efficiencyvalues are used to predict the RF sensitivity and read range of the RFIDtag.

The antennas provided by embodiments of the present disclosure may befabricated in one embodiment using flex PCB materials. Electricalconnections between the bumps of the integrated circuit and the antennaallow the antenna and integrated circuit to be electrically coupled.

FIG. 30A provides an illustration of an antenna arrangement inaccordance with another embodiment of the present disclosure. In thisslot tag antenna arrangement 3000, antenna 3000 may comprise one or moretuning slot lengths, 3002A and 3002B. Wherein IC 2100 couples to theantenna via bond pad connections. FIG. 30B depicts a second arrangementof a slot tag design. Wherein the longitudinal slots 3006A and 3006Bform an H pattern with lateral slot 3008. This slot tag may comprise asingle layer of copper on kapton laminated with transfer tape onto a PETcore. This PET core may comprise one or more layers laminated withtransfer tape such as the overall thickness of the dielectric layer maybe about 40 mil.

FIGS. 31 and 32 are views of an RFID pressure sensing tag 3100 inaccordance with an embodiment of the present disclosure. Pressuresensing tag 3100 is a passive RFID tag, which includes a sensor, thesensor having a variable sensor impedance, and IC 2100. The sensorimpedance varies. In one embodiment conductive plate 3104 is locatedproximate to a tuning circuit 3102. When an external pressure is appliedto the conductive plate, the separation between conductive plate 3104and tuning loop 3102 is reduced causing an impedance change. Theimpedance of the tuning circuit in the processing module coupled to thesensor then produces an output, a sensor code, representative of thepressure applied. This data may be stored within a memory circuit of IC2100 or transmitted to an external reader by the wireless communicationmodule of IC 2100.

Conductive plate 3104 may sit on a compressible material space 3106 asshown in the cross section of FIG. 32. The presence and relativemovement of conductive plate 3104 reduces the inductance of the tuningloop 3102. This causes the tuning module of IC 2100 to generatedifferent sensor codes to compensate for the impedance change. In oneembodiment, compressible material space 3106 has a substantially linearcompression between 25 and 50% compression.

FIG. 33 is a view of an RFID moisture or humidity sensing tag 3300 inaccordance with an embodiment of the present disclosure. Moisture orhumidity sensing 3300 is a passive RFID tag, which includes a sensor,the sensor having a variable sensor impedance, and IC 2100. The sensorimpedance varies as the coupling of interdigitated capacitor 3304responds to environmental changes. In one embodiment interdigitatedcapacitor 3304 is located proximate to a film 3306 applied aboveinterdigitated capacitor 3304. Film 3306 may be a material having anaffinity for water (i.e. moisture or humidity) or other fluids. Thesefluids may include CO, CO2, Arsenic, H2S or other known toxins or gasesof interest. When film 3306 absorbs a fluid such as those describedpreviously, the dielectric constant proximate to the interdigitatedcapacitor 3304 changes causing an impedance change. The impedance of theinterdigitated capacitor 3304 sensed by the processing module coupled tothe sensor then produces an output, a sensor code, representative of theabsorbed material within film 3306. This data may be stored within amemory circuit of IC 2100 or transmitted to an external reader by thewireless communication module of IC 2100.

FIGS. 34A and 34B are views of a folded RFID tag 3400, including antenna3402 comprising a radiating element, the radiating element comprising afirst wing 3402A and a second wing, the second wing divided into aproximal section 3402B and a distal section 3402C, the distal section3402C folded onto the proximal section 3402B, and the first wing 3402Afolded onto the folded second wing, the distal section 3402C of thesecond wing capacitively couples to the proximal section 3402B and thefirst wing 3402A. These sections are folded about a PCB core.

FIG. 35 is a block diagram of one arrangement of a self-tuning engine tosupport the reporting of several stimuli with multiple passive RFIDsensors using an antenna impedance sensing mechanism in accordance withembodiments of the present disclosure. Module 3500 includes antennaports 3502A-N, self-tuning engine 3504, processing unit 3506, referenceinput module 3508 and power harvesting module 3510. A number of antennaports 3502A-N passively sense stimuli through changing antennainductance as previously discussed. The self-tuning engine 3504 adjustsa variable capacitance 3512A-N in response to the inductance sensed asADC 3514A-N wherein decision module 3516A-N directs feedback to adjustthe value of variable capacitance 3512A-N and produce a code reported toprocessing unit 3506. This sensor code reflects the sensed stimulirelative to the antenna inductor 3518A-N. The stimuli sensed may be anycombination of stimuli sensed by the changing inductance of the antenna(i.e. pressure, moisture, proximity etc.) Processing unit 3506 iscoupled to the self-tuning engine 3504 and other potential referenceinputs such as those provided by reference block 3520. Reference block3520 allows the processing unit to compensate for external elementssensitive to external stimulus with an input to processing unit 3506.One such example may be where an external element is sensitive to acondition such as temperature, in this example reference block 3520provides a reference signal 3522 for the processing unit 3506. The blockas a whole may be powered by a power harvesting engine 3510 to supplyon-chip power needs.

FIG. 36 is a block diagram of a self-tuning engine in accordance withembodiments of the present disclosure. Self-tuning engine 3600 includesan antenna 3602, a variable capacitance or varactor module 3604, a clockacquisition and data conversion module 3606, a monitoring module 3608, adecision module 3610, processing module 3614, and a clock module 3612.

Varactors are basically voltage-controlled capacitors. Varactors areimplemented in various forms, for example as discrete components, inintegrated circuits, in MEMS (microelectro-mechanical systems).Varactors are widely used in RF circuits as tuning elements. Examples oftwo MOS varactors can be seen in FIGS. 37A and 37B their capacitance vs.voltage plots (CV curve) in FIGS. 38A and 38B.

FIGS. 37A and 37B are representations of MOS Varactors, where FIG. 37Aillustrates an inversion MOS varactor, and FIG. 37B illustrates anaccumulation MOS varactor. The varactor shown in FIG. 37A is aninversion type MOS varactor and is built by connecting bulk of thetransistor to the highest positive voltage available in the circuit andby connecting drain and source terminals to each other. Thus, transistordoes not enter accumulation region, and capacitance is determined bygate and S/D (Source/Drain) terminals. This inversion MOS varactorexhibits steep CV curve behavior as shown in FIG. 38A.

The varactor shown in FIG. 37B is an accumulation type MOS varactor.This varactor is similar to a NMOS transistor with the drain (D) andsource (S) terminals are connected to each other. The bulk of thetransistor is n-type not a p-type and is not connected to a highervoltage as was the case for an inversion type MOS varactor. Byincreasing the gate voltage applied, more electrons are attracted to thegate, so capacitor value increases. This accumulation type MOS varactorexhibits the CV curve behavior as shown in FIG. 38B.

For a specific process, the maximum and minimum capacitance valuesachieved are same for an inversion and accumulation type varactors. Asthe mobility of electrons are higher than hole mobility and CV curve ofan accumulation type varactor is smoother, accumulation type MOSvaractors are often preferred. Quality factors of MOS varactors areusually in the range of 50 to 80 at 1 GHz and tuning ranges are 40 to60%.

FIG. 39 illustrates an embodiment of the self-tuning engine provided byembodiments of the present disclosure along with the varactors that aredriven by the tuning circuitry (referred to as MMS engine). Thevaractors in this embodiment are enhancement MOS varactors. In oneembodiment, the engine generates 5 bits of sensor code (also referred toas MMS code) that are then converted to 16 bits (i.e. n=16) ofthermometer codes. Each bit of the thermometer code drives one varactorunit. In this embodiment there are a total of 16 varactor units (eachunit is a varactor on its own). Each code can be either VDDA (a highvoltage) or VSSA (a low voltage signal). The antenna ports; ANTP andANTN, are set at a voltage value of VDDA/2.0 under normal operation.Looking at this from the varactor perspective, the Gate of each of the16 varactor units will always be at VDDA/2.0V with respect to Bulk,while the S/D, (Source/Drain), connection of each of the 16 varactorunits will be set to VDDA or 0V with respect to Bulk, depending on thesensor code generated. Hence, each of the 16 varactor units will be setto either its min capacitance or max capacitance value. The totalcapacitance of the varactor structure is the sum of these min/maxvalues. This implementation is referred to here as a digitalimplementation of an embodiment of the self-tuning engine provided byembodiments of the present disclosure.

FIG. 40 is a Varactor's CV curves for different V_(SD) voltages. In oneembodiment, FIG. 40 shows the varactor's CV curves for different S/Dvoltages; 0V (4002) and 2V (4004). For each curve, the gate voltage withrespect to Bulk is swept from −2V to +4V and for each sweep, thecapacitance of the varactor is plotted.

Under normal operation, the Gate is always sitting at 1V, hence theblack and dotted line in FIG. 40. When the S/D is set to 0V, the redcurve is used and the varactor's capacitance is equal to Cmax=243 fF.When the S/D is set to 2V, the blue curve is used and the varactor'scapacitance is Cmin=93 fF.

Another embodiment is shown in FIG. 41. In this embodiment, the 16 unitsof varactors from the embodiment of FIG. 39 are replaced with one unitthat has the size of 16 units. The varactor in this embodiment is anenhancement MOS varactor. A digital to analog converter (DAC or D2A) isused to convert the output of the self-tuning engine to an analog signalthat drives the source/drain (S/D) connection of this new unit. Eitherthe 5-bit sensor code or its 16-bit thermometer code equivalent can beused to generate the analog signal. DACs are well known circuits in theart and there are many suitable DAC circuits that can be used for thispurpose. This implementation is referred to here as the Analogimplementation.

FIG. 42 is a graph of a CV curve for the varactor when S/D voltages areswept in accordance with embodiments of the present disclosure. FIG. 42shows the new CV curve 4202 for the varactor when the source/drain (S/D)voltage is swept. Notice that it has a negative slope. As can be seenfrom FIG. 42, rather than having 16 discrete steps of varactor values(as with the digital implementation in FIG. 40), the analog controlvoltage provides a continuum of capacitance values. The design providesmore flexibility that the digital implementation. The same structure canbe used in various implementations with different resolutions of sensorcode and/or DAC. Some of the advantages of the analog implementation areease of implementation and layout of the varactor, savings in area androuting complexity and modularity of the design.

Embodiments of the present disclosure include RFID sensors with dynamictuning circuits that are capable of varying the value of a variablecapacitor in order to match the impedance of a variable impedanceantenna. The capacitor value can be varied in several ways, inparticular a digital implementation and analog implementation. Theanalog implementation utilizes the sensor code from the tuning circuitand converts that sensor code, via a digital to analog converter, intoan analog signal that is used to vary the voltage on the gate of, forexample an MOS varactor.

Returning to FIG. 36, the clock acquisition and data conversion module3606 will sense a voltage associated with the variable capacitance orvaractor 3604 that may change as a function of antennae impedancewherein the impedance is changed based on environmental stimulus orother like conditions. Monitoring module 3608 may monitor phase andamplitude or other qualities associated with the data collected by clockand data conversion module 3606. This information is then provided toprocessing module 3614 which in conjunction with decision module 3610may place capacitors 3616 A through N in service within the variablecapacitance or varactor 3604 in order to maximize power transfer orother like considerations with antennae 3602. The manipulation of thevaractor 3604 will relate to a sensor code as discussed previously orother like signal. Clock 3612 provides a clock input to the variousmodules within Engine 3600 such that the data acquisition and theactions of the various processing modules may be coordinated.

Embodiments of the present disclosure may provide a passive RFID sensor(IC chip, antenna, and package) such that once an event of interest hasoccurred, the structure of the antenna and package may change itscharacteristics in an irreversible manner. FIG. 43 provides a flow chartof one such embodiment. In Block 4302, a passive RFID sensor, such as anantenna may be inlaid within the structure wherein the antenna may altera physical characteristic such as impedance when exposed to a suddenforce. For example, an antenna may be wrapped around a glass or otherstructure. The original impedance value may be recorded and stored forcomparison in block 4304. In block 4306, the impedance value may be readon an ongoing basis wherein when the impedance value or a codeassociated with the impedance value changes, that change signals thatthe event of interest may have occurred. Such an event may be when anobject on which the passive sensor is mounted has been dropped.

Embodiments of the present disclosure may also provide a passive tirepressure monitoring system wherein tire pressure or pressure in anyspace may be sensed without the need for a local on-board battery.Further the need for complicated circuitries to sense acceleration maybe avoided. A RFID reader such as that discussed with reference to theprior FIGs. may automatically find and determine the location of eachtire by a strength difference associated with the signal. FIG. 44depicts one such embodiment wherein four unique tire pressure monitoringsensors are individually located wherein the RFID Reader 4402 may haveone or more Antennas 4404 and 4406 wherein signal strength differencesbetween signals read from individual passive tire pressure monitoringsensors 4408A-D allow the RFID reader to determine which passive sensoris associated with which tire. As shown in FIG. 44, each tire is locatedat a different difference from the antennas 4404 and 4406. Althoughamplitude may be used to determine tire position, other signalproperties may be used as well.

FIG. 45 is a block diagram of a moisture sensor used to monitor thelevel within a reservoir or tank. Reservoir 4500 may be a wash tubassociated with a dishwasher or washing machine, wherein the primaryreservoir 4500 is filled with a fluid 4502 having a level 4504 withinthe reservoir. Level 4504 is measured using a passive RFID sensor 4506inductively coupled to the fluid 4508. FIG. 45 shows reservoir 4500having a sensor tube 4510 communicatively coupled to the reservoirwherein a passive RFID Sensor 4506 is placed on the outer surface of thesensor tube 4510. Sensor tube 4510 may be capped although in otherembodiments sensor tube 4510 may run the entire height of reservoir4500. Capping sensor tube 4510 substantially prevents the tube fromcompleting filling with fluid. On an external surface of sensor tube4510 a tapered spacer 4512 may be placed between sensor tube 4510 andpassive RFID Sensor 4506. This spacer decreases in width as sensor tube4510 height increases. This makes the passive RFID sensor 4506 moresensitive as the fluid level rises with improved inductive couplingbetween the fluids contained within the sensor tube and the passive RFIDsensor tag.

The fluid height in the reservoir in FIG. 45 is proportional to thefluid height in the center tube. The height may differ especially whenthe sensor tube 4510 is capped. This proportionality depends on thecompressible air volume in the sensor tube and, the atmospheric pressureto which the reservoir is exposed and the height of the fluid within thereservoir. Experiments depicted in FIG. 46 illustrate the relationbetween fluid height and a sensor code generated by a sensor on a singlesensor tube in FIG. 45.

FIG. 47 depicts a second embodiment of a sensor tube such as thatdisclosed in FIG. 45 wherein sensor tube 4700 contains both a verticalsegment 4702 of the tube and a horizontal segment 4704 of the tubewherein sensor tags 4706 and 4708 are located within protectiveenclosures, 4710 and 4712 on the vertical and horizontal segments ofSensor Tube 4700. This two-sensor system allows the horizontal tag tobecome quickly coupled to fluid within the horizontal segment of thesensor tube as the reservoir begins to fill. The vertical tag is coupledmore slowly as the reservoir fills. This will allow the empty reservoirto be readily sensed by both the vertical and horizontal sensor tags.The vertical tag again couples more slowly and will do so after apositive report from the horizontal tag. Variance in the inductance ofthe horizontal tag may be used as a reference to account for changes inconductance of the fluid of the sensor tube caused such as detergents orother dissolved substances within the fluid. FIG. 48 provides analternate embodiment of a sensor tube having more than one sensor tag.In this embodiment Sensor Tube 4800 has two vertical sensor tags 4802and 4804 a secondary tube 4806 that runs the full height of thereservoir and may be open to air. The primary sensor tube 4808 is sealedhaving an air chamber. The secondary tube 4802 couples to the primarytube below the height of the upper vertical sensor tag 4802. The lowervertical sensor tag 4804 when submerged may provide a signal that allowsa more accurate reading of the height by accounting for changes inconductivity within the fluid of the sensor tube 4808.

Embodiments of the present disclosure provide a passive radio frequencyidentification (RFID) sensor. This passive RFID sensor includes anantenna, a processing module, and a wireless communication module. Theantenna has an antenna impedance that may vary with an environment inwhich the antenna is placed. The processing module couples to theantenna and has a tuning module that may vary a reactive componentimpedance coupled to the antenna in order to change a system impedance.The system impedance including both the antenna impedance and thereactive component impedance. The tuning module then produces animpedance value representative of the reactive component impedance. Amemory module may store the impedance value which may then later becommunicated to an RFID reader via the wireless communication module.The RFID reader may then exchange the impedance value representative ofthe reactive components of impedance with the RFID reader such that theRFID reader or another external processing unit may process theimpedance value in order to determine environmental conditions at theantenna. These environmental conditions may include but are not limitedto temperature, humidity, wetness, or proximity of the RFID reader tothe passive RFID sensor.

In another embodiment, a conductor or transmission line couples theantenna to the processing module allowing the antenna to be positionedremotely or offset from the processing module. In yet anotherembodiment, a sensor having the sensor impedance that varies with theenvironment may be coupled to the processing module wherein the sensorimpedance may be sensed via a sensor tuning module in much the same waythat the antenna impedance is sensed and since a reactive componentimpedance is determined and a value representative of the impedance isproduced which may again be transmitted to an RFID reader for externalprocessing.

In one embodiment, the sensor is offset from the processing module via aconductor or transmission line. In one particular embodiment the sensoris positioned within a cavity offset from the processing module whereinthe cavity is impervious to radio frequency signals. This sensor may bean open circuited transmission line where the open circuitedtransmission line only introduces a capacitance when liquids are presentproximate to the open circuit transmission line. The capacitance changesin such an example may change with the volume of liquid proximate to theopen circuited transmission line. This is extremely useful when placingliquid or water sensors within cavities such as those contained within avehicle chassis or when the cavities are prone to fluid incursion. Thisallows the sensor to be offset from the processing module where theenvironment to be sensed is hostile to the processing module.

In another embodiment, the sensor may be an interdigitated capacitorwherein the capacitor's impedance changes in response to moisture, i.e.humidity proximate to the interdigitated capacitor. In yet anotherembodiment, the sensor may be a conductive plate proximate to a tuningfork wherein the conductive plate is separated from the tuning fork by acompressible insulating material wherein an external pressure applied tothe conductive plate changes an impedance or inductance of the tuningcircuit. In the case of the interdigitated capacitor, the impedance maychange in response to an environmental dialectic constant change in theenvironment proximate to the interdigitated capacitor. This may occurwhen different gasses or fluids proximate to the sensor involve a changein dielectric constant at the sensor as may be caused by changing gas.Thus in one embodiment the passive RFID sensor may be used to detect anenvironment toxin such as CO, CO2, arsenic, hydrogen sulfide or otherhazardous chemicals.

The passive RFID sensor may also include an RFID power harvesting moduleoperable to receive energy form the RFID reader and power the passiveRFID sensor with the received power. The processing module may determinehow much of this energy is to be consumed by the passive RFID sensor anddivert any remaining energy to a reservoir power harvesting element.Additionally, the memory module may store identification information forthe passive RFID sensor wherein the identification information may beprovided with the impedance values associated with the antenna or aseparate sensor and be provided to the RFID sensor for furtherprocessing. Additionally, a time stamp may be applied to thisinformation. This may allow the RFID reader to generate an alarm signalbased on certain measured environmental conditions.

Thus it is apparent that embodiments of the present disclosure haveprovided an effective and efficient method and apparatus for sensingchanges to an environment to which the RFID tag is exposed.

Those skilled in the art will recognize that modifications andvariations can be made without departing from the spirit of the presentdisclosure. Therefore, we intend that embodiments of the presentdisclosure encompass all such variations and modifications as fallwithin the scope of the appended claims. The system controllers orprocessors may comprise a microprocessor may be a single processingdevice or a plurality of processing devices. Such a processing devicemay be a microprocessor, micro-controller, digital signal processor,microcomputer, central processing unit, field programmable gate array,programmable logic device, state machine, logic circuitry, analogcircuitry, digital circuitry, and/or any device that manipulates signals(analog and/or digital) based on operational instructions. Memory maycouple to the microprocessor in the form of a single memory device or aplurality of memory devices. Such a memory device may be a read-onlymemory, random access memory, volatile memory, non-volatile memory,static memory, dynamic memory, flash memory, cache memory, and/or anydevice that stores digital information. Note that when themicroprocessor implements one or more of its functions via a statemachine, analog circuitry, digital circuitry, and/or logic circuitry,the memory storing the corresponding operational instructions may beembedded within, or external to, the circuitry comprising the statemachine, analog circuitry, digital circuitry, and/or logic circuitry.The memory stores, and the processing module executes, operationalinstructions corresponding to at least some of the steps and/orfunctions illustrated in the FIGs.

As one of average skill in the art will appreciate, the term“substantially” or “approximately”, as may be used herein, provides anindustry-accepted tolerance to its corresponding term. Such anindustry-accepted tolerance ranges from less than one percent to twentypercent and corresponds to, but is not limited to, component values,integrated circuit process variations, temperature variations, rise andfall times, and/or thermal noise. As one of average skill in the artwill further appreciate, the term “operably coupled”, as may be usedherein, includes direct coupling and indirect coupling via anothercomponent, element, circuit, or module where, for indirect coupling, theintervening component, element, circuit, or module does not modify theinformation of a signal but may adjust its current level, voltage level,and/or power level. As one of average skill in the art will alsoappreciate, inferred coupling (i.e., where one element is coupled toanother element by inference) includes direct and indirect couplingbetween two elements in the same manner as “operably coupled”. As one ofaverage skill in the art will further appreciate, the term “comparesfavorably”, as may be used herein, indicates that a comparison betweentwo or more elements, items, signals, etc., provides a desiredrelationship. For example, when the desired relationship is that signal1 has a greater magnitude than signal 2, a favorable comparison may beachieved when the magnitude of signal 1 is greater than that of signal 2or when the magnitude of signal 2 is less than that of signal 1.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. The corresponding structures,materials, acts, and equivalents of all means or step plus functionelements in the claims below are intended to include any structure,material, or act for performing the function in combination with otherclaimed elements as specifically claimed. The description of the presentdisclosure has been presented for purposes of illustration anddescription, but is not intended to be exhaustive or limited to theinvention in the form disclosed. Many modifications and variations willbe apparent to those of ordinary skill in the art without departing fromthe scope and spirit of the invention. The embodiment was chosen anddescribed in order to best explain the principles of the invention andthe practical application, and to enable others of ordinary skill in theart to understand the invention for various embodiments with variousmodifications as are suited to the particular use contemplated

What is claimed is:
 1. A passive radio frequency identification (RFID) sensor, comprising: at least one antenna, wherein each at least one antenna has an antenna impedance, the antenna impedance operable to vary within an environment in which the antenna is placed; a processing module coupled to the antenna, the processing module comprising: a tuning module coupled to the antenna, the tuning module operable to: vary a reactive component impedance coupled to the antenna in order to change a system impedance, the system impedance comprising the antenna impedance and the reactive component's impedance, produce an impedance value representative of the reactive component's impedance; a memory module operable to store the impedance value representative of the reactive component's impedance; and a wireless communication module coupled to the processing module, the wireless communication operable to communicate with an RFID reader, the wireless communication module operable to exchange the impedance value representative of the reactive component's impedance with the RFID reader.
 2. The passive RFID sensor of claim 1, wherein the RFID reader communicates the impedance value representative of the reactive component's impedance to a processing unit, the processing unit operable to determine at least one environmental condition at the antenna from the impedance value representative of the reactive component's impedance.
 3. The passive RFID sensor of claim 1, wherein the antenna impedance changes with pressure, temperature, humidity, wetness, and/or proximity.
 4. The passive RFID sensor of claim 1, wherein the reactive component impedance comprises a varactor driven by an analog signal from the tuning module, the analog signal enabling a continuous reactive component impedance response.
 5. The passive RFID sensor of claim 1, further comprising: at least one additional antenna, the at least one additional antenna having a second impedance, the second impedance operable to vary with an environment in which the sensor is placed; the tuning module coupled to the at least one additional antenna, the tuning module further comprising: at least one additional variable reactive component coupled to the at least one additional antenna in order to change a system impedance, the system impedance comprising the at least one additional antenna impedance and the at least one additional variable reactive component's impedance, the tuning module operable to: vary the at least one additional reactive component's impedance; produce an impedance value representative of the variable reactive component's impedance; store the impedance value representative of the variable reactive component's impedance.
 6. The passive RFID sensor of claim 5, wherein the at least one antenna are proximate to an external surface of a sensor tube, the sensor tube operable to contain a fluid having a variable level and or conductivity.
 7. A passive radio frequency identification (RFID) sensor, comprising: a first inductive loop, wherein the first inductive loop has a first impedance, the first impedance operable to vary in response to an environmental parameter proximate to the first inductive loop; a processing module coupled to the first inductive loop, the processing module comprising: a tuning module coupled to the first inductive loop, the tuning module operable to: vary a first reactive component impedance coupled to the first inductive loop in order to change a system impedance, the system impedance comprising the first inductive loop impedance and the first reactive component's impedance, produce a first impedance value representative of the first reactive component's impedance; a memory module operable to store the first impedance value representative of the reactive component's impedance; and a wireless communication module coupled to the processing module, the wireless communication operable to communicate with an RFID reader, the wireless communication module operable to exchange the first impedance value with the RFID reader.
 8. The passive RFID sensor of claim 7, further comprising: a second inductive loop, wherein the second inductive loop has a second impedance, the second impedance operable to vary in response to an environmental parameter proximate to the second inductive loop; the processing module coupled to the second inductive loop, the processing module operable to report the second impedance value representative of the reactive component's impedance.
 9. The passive RFID sensor of claim 7, wherein the first inductive loop impedance changes with pressure, temperature, humidity, wetness, and/or proximity.
 10. The passive RFID sensor of claim 7, wherein first reactive component impedance comprises a varactor.
 11. A passive radio frequency identification (RFID) sensor, comprising: a first inductive loop, wherein the first inductive loop has a first impedance, the first impedance operable to vary in response to environmental parameter(s) proximate to the first inductive loop; a second inductive loop, wherein the second inductive loop has a second impedance, the second impedance operable to vary in response to environmental parameter(s) proximate to the second inductive loop; a processing module coupled to the first inductive loop and the second inductive loop, the processing module comprising: a tuning module coupled to the first inductive loop and the second inductive loop, the tuning module operable to: vary a reactive component impedance coupled to each inductive loop in order to change a system impedance, the system impedance comprising each inductive loop impedance and the reactive component's impedance, produce an impedance value representative of each reactive component's impedance; a memory module operable to store the impedance value representative of the reactive component's impedance; and a wireless communication module coupled to the processing module, the wireless communication operable to communicate with an RFID reader, the wireless communication module operable to exchange the impedance values with the RFID reader.
 12. The passive RFID sensor of claim 11, wherein the RFID reader communicates the impedance value representative of the reactive component's impedance to a processing unit, the processing unit operable to determine at least one environmental condition at each inductive loop from the impedance value representative of the reactive component's impedance.
 13. The passive RFID sensor of claim 11, wherein each loop inductance impedance changes with pressure, temperature, humidity, wetness, and/or proximity.
 14. The passive RFID sensor of claim 11, wherein the reactive component impedance comprises a multiplexed impedance.
 15. The passive RFID sensor of claim 11, wherein the reactive component impedance comprises a variable capacitance coupled to each inductive loop.
 16. The passive RFID sensor of claim 11, wherein the reactive component impedance comprises a varactor coupled to each inductive loop.
 17. The passive RFID sensor of claim 11, wherein the reactive component impedance comprises a varactor coupled to each inductive loop, the varactor driven by an analog signal from the tuning module, the analog signal enabling a continuous reactive component impedance response.
 18. The passive RFID sensor of claim 11, wherein the inductive loop comprises an interdigitated capacitor, wherein the interdigitated capacitor's impedance changes in response to moisture (humidity) proximate to the interdigitated capacitor.
 19. The passive RFID sensor of claim 11, wherein the inductive loop comprises a conductive plate proximate to a tuning circuit, wherein an external pressure applied to the conductive plate changes an impedance (inductance) of the tuning circuit.
 20. The passive RFID sensor of claim 11, wherein the inductive loop comprises an interdigitated capacitor, wherein the interdigitated capacitor's impedance changes in response to a change in an environmental dielectric constant proximate to the interdigitated capacitor. 